1. Field of the Invention
This application relates generally to digital image scaling. More specifically, this application relates to a raw domain image scaling system and method that provides flexible implementation of multiple scaling factors.
2. Description of Related Art
In a digital image capturing system, it is common that the resolution requirement for still image capture is higher than that of video output. In such a case, a high-resolution camera capable of supporting the resolution requirement of still image capture is often used. When the camera is used in video mode to produce a video output stream, the data is scaled down to suit the resolution of the video output. For example, a particular image sensor with 8 million pixels (megapixels or MP) can give 8 MP still images. For the same image sensor in video mode, a resolution of 1080×1920 pixels, or approximately 2 MP, may suffice to produce 1080p high-definition (HD) video. Therefore, in 1080p video mode, the image data is scaled down 3× both vertically and horizontally to provide the desired output resolution.
This scaling may be performed either in the raw domain or the RGB domain. An advantage of scaling the image data in the raw domain to the desired video resolution is that it reduces the number of pixels that must be processed through the system. As a result, a majority of processing blocks in an image pipeline or post-processing section can be operated at a lower clock rate than the clock rate required to support full-resolution processing. Operating the processing section at a lower clock rate has significant advantages in reducing electromagnetic interference and reducing power consumption of the system. These advantages are especially valuable in such applications as mobile imaging.
The raw scaling step can be interpreted as a filtering procedure which calculates a weighted average of selected input raw pixels to produce the values of the output raw pixels. The weights used and the pixels selected in the weighted averaging has a large impact on the quality of the output image. To optimize the quality of the output images, a raw domain image scaling method called a “Bayer-consistent raw scaling” (BCRS) method may be used.
When BCRS or another raw domain image scaling method is implemented in an image sensor, it is necessary to incorporate additional circuits, either in the analog domain or in the digital domain (or both), to generate the scaled raw pixels. As a result, it can become necessary to increase the size of the circuit inside the sensor and increase the power consumption of the sensor.
Accordingly, there is a need to incorporate BCRS or another raw domain image scaling method in the sensor in a manner so as to optimize the quality, and match the resolution of the sensor and display, without resulting in a significant increase in circuit complexity and power consumption.
In a structure wherein the raw image scaling filter (RISF) is integrated with a decimation filter of a sigma-delta ADC (an “ADC filter” or “ADCF”), efficient and low power implementation in imaging systems may be achieved. However, such a structure requires that the ADC column sharing structure either match the scaling factor, or be an integer multiple thereof. That is, the scaling factor (e.g. 1/3 X) should match that of the hardware (e.g. three-column sharing structure), or is related to the hardware architecture (e.g. 6-column sharing structure) by an integer multiplier. While such a design is useful for systems that use a fixed scaling factor (e.g. an 18 MP image sensor downscaled by 1/3 X to support 1080P video), the implementation is restrictive in systems where multiple scaling factors need to be supported, or in cases where column sharing is selected for other reasons (e.g. frame rate) that end up being not an integer multiple of the scaling factor. Thus, there is a need to incorporate raw scaling in the sensor in a manner that provides the flexibility to support multiple scaling factors, in addition to the above needs.